Twisted pair communications cable is used for high frequency signal transmission, typically in plenum areas of buildings. The cable is composed of twisted pairs of polymer-insulated conductors, covered by a polymer jacket. Usually the cable contains multiple twisted pairs separated from one another by a spline having a cruciform cross-section section, all being contained within a common polymerjacket. For flame retardency and smoke resistance, in case a building fire occurs, the polymer insulation is fluoropolymer. In the case of multiple twisted pairs within a single cable, a small number of the polymer insulations can be polyolefin, which by itself is both flammable and emits smoke when burning. The combination of fluoropolymer insulation as the predominating insulation, together with polyolefin insulation is acceptable under some building circumstances.
One requirement of the twisted pair polymer-insulated conductors is the transmission of electrical signals with little to no signal loss. One mechanism of signal loss is the absorption of signal energy by the polymer insulation. This absorption increases as the mass of the polymer insulation increases. Thus, it is common that thin insulation thicknesses are used, typically no greater than about 20 mils (500 μm), usually no greater than about 12 mils (300 μm). Foamed insulations have been used to reduce the mass of polymer in the insulation, and indeed this reduces the energy absorption (capacitance) of the polymer insulation. The problem with foamed insulations, however, has been that the foamed insulation is compressible by the twisting operation which combines (twins) two polymer-insulated conductors together. In the course of being twisted together, the surfaces of the polymer insulations are forced together. The magnitude of the force varies with twisting equipment and the tightness of the twist, i.e. number of turns per unit of length, e.g. /ft or /m. The result of this force compressing the surface of the foamed insulation is to decrease its thickness, resulting in decreased dielectric property (decreased impedance) between the two insulated conductors of the twisted pair at the location of insulation compression. To compensate for this undesirable loss in insulation thickness, the polymer-insulated wire manufacturer must increase the thickness of the foamed polymer insulation in the extrusion foaming process of applying the insulation to the conductor. This detracts from the advantage of using foamed insulation instead of solid (unfoamed) insulation and creates difficulties in fitting the foamed-insulated twisted pair cables into small spaces and prevents the utilization of existing connector sizes.
The problem is how to make the substitution of foamed insulation for solid insulation without creating the disadvantage of greater compressibility of the foamed insulation arising from the twisting process.